What are people doing when they try to make images of Christ? Forget the obvious answers for a moment; there have never been utterly obvious answers in the history of Christian art, even though we might think there have.

The very earliest representations have a sort of anonymous quality: the young man staring into the distance with a lamb over his shoulders, one of the commonest early Christian images, could be a routine piece of classical decoration. The simple catacomb paintings of gospel miracles show, again, an anonymous youth, marked out only by a kind of magic wand, in scenes that are anything but self-explanatory.

All these images inhabit an imaginative world in which an ordinary figure is put into a context where it is clear that some sort of extra meaning attaches to him - but there is no clear way of saying what that extra is.

As has sometimes been said, there is something prosaic and uncanny about these depictions. What they say to us is not so far from what they must have said to the non-Christian Roman viewer: here is something important, and you will know what it is only by sharing the life of the people who made this image. The image will not tell you its own importance. You are invited to sit alongside people for whom this matters and discover why. If you don't, the image simply remains. So all you see is an anonymous youth? Then that's all you see.

Kierkegaard wrote in the 19th century that there was a sense in which all Christian theology and proclamation boiled down to someone saying to someone else: "Once God walked the streets as a human being." No story, no way of identifying where God is; we simply know that it was true and so everything is different.

I'm not convinced that Kierkegaard said all that needs saying - but he encapsulates something of what the catacomb paintings of Jesus say. Here is an anonymous figure, of which we say that his actions change our frame of reference; how? The picture will not tell you because that would be to suggest that the change can be represented - and so would cease to be the kind of change we are trying to talk about.

Some of the history of Christian art is about the tension between recognising that the change associated with Jesus is incapable of representation and recognising that for the change to be communicable it must in some way be represented. The violent controversies of the seventh and eighth centuries over iconoclasm reflect this conundrum, and some of the most subtle and interesting strands in Greek theology were deployed to secure a resolution, whose effects are still to be seen in classical eastern iconography.

What I have described as anonymity is one strategy to cope precisely with the ingrained tensions of representing a divine event in human narrative record. But another strategy is irony, the irony - regularly missed or misread - of much of this traditional eastern iconic representation of Jesus.

Here is a figure in the imperial position, enthroned at the "royal" end of the meeting hall; but he is a long-haired, bearded exotic, dressed as a teacher of philosophy, usually pointing to a book. What is "royal" or imperial in the order of the universe? Wisdom. Who is the emperor's emperor? A wandering scholar. It is another way of representing a change, putting a figure in a position that is in some way uncanny.

Once the irony has been lost or forgotten, once a tradition has become "obviously" the way to depict Christ, the image can retain power of a kind, but its witness to change becomes less evident. Or it can be systematically robbed of irony by modification: the late medieval version of the eastern icon of Christ that shows him robed as bishop and crowned as emperor flattens out some thing of the complexity of earlier types. Western pictures of Christ the king or of the triple-crowned God the father (Van Eyck) also risk losing the ironic, though the crowned and vested Christ on the cross sometimes retains it.

There are artists who have tried to show the change of the world around Jesus in more narrative and dramatic ways. Raphael's Transfiguration comes to mind, and the resurrection picture that forms part of Grünewald's Isenheim Altarpiece with its unsparing crucifixion. Why are these images so uncomfortable? Surely because the picture tries to "include" the difference, to render it in a way that can say only, "What is different is that a familiar experience is raised to the highest possible degree by Jesus"; so let us have blinding brightness and levitation, an extremism of effect and emotion.

This is a personal view, obviously, but it is why I find baroque religious art both brilliantly inviting and exhilarating, and spiritually flat. Something of the difference that is God is absent, and what remains is a world of extremity that may or may not precipitate a beholder into the radical presence of what is not of this world. This is an art that eschews irony and so denies itself one of the subtler tools for evoking what is distinctive about the figure of Jesus.

The same absence of irony is even more conspicuous in later modern religious art - the German Nazarenes, the pre-Raphaelites, the historical realism of Tissot or Hole. What has come to matter is the record of things that have happened; but the very mood of the painting declares that this is a story you know. The figure of Jesus is anything but anonymous: robed and radiant, calm, stately, how could anyone fail to see that he matters, that he is holy?

The style renders visible the obviousness of religious sentiment of a certain kind, and so makes practically unthinkable any perception other than that already familiar. That is not to say it cannot operate as religious art of a kind, as what you might call casual reinforcement of shared piety; but it fails, once again, as a depiction of difference or distinctiveness, and so as a depiction of the divine importance, the reference-changing character, of Jesus.

All in all, it is no surprise the contemporary depiction of Christ is so complex a task. In some areas, the problem is, if not solved, at least modified by the requirements of a genre: the Stations of the Cross, for example, which have produced some powerful modern renderings, like those of Peter Howson. These are traditionally figural, but there are also fine instances of something far less representational. Here it is the associated practice that displays transforming meaning. Works that do not have this clear liturgical role face taxing challenges. But it is intriguing to see how variously the "uncanny" has been handled.

Mark Wallinger's work stands out here, especially Ecce Homo; this is a piece that still provokes debate, and that I find myself both admiring and querying. But its installation in Trafalgar Square produced, I think, exactly the sort of double-take that catacomb art induces (and probably induced at first): here is an ordinary representation that clearly has extraordinary significance, but a significance that does not show itself as an element in the actual physical composition.

The ironic use of familiar or half-familiar images casts light on the performance art of Michael Gough, walking the streets dressed as the Jesus of representational cliche; but also on the use of fractured or otherwise distorted reproductions of actual classical images in collage or comparable mode.

Yuri Titov, active in the 1960s and 1970s, used pictures of broken or vandalised icons of Christ against stylised urban Soviet backgrounds to say something about the persistence of the image in and through apostasy and social devastation. Something even beyond irony is evoked by the work of Roger Wagner, with his fusion of Jewish and Christian symbols and narratives with the cooling towers of Didcot power station - Jewish victims of the Shoah wandering in the neighbourhood of a distantly seen, conventionally depicted crucifixion, the background dominated by the immense towers exuding gas, arranged in the pattern of the ceremonial candlestick, the menorah that gives this 1993 painting its title. This is very dense imagining indeed, but it manages a representation of the creatively and theologically uncanny that is haunting.

The contemporary artist trying to represent Jesus is not, then, confined to modernistic recycling of iconic forms; the challenge is to create a frame of visual reference that prompts the question of how and why a whole landscape is altered by the presence of the single provoking figure. Presumably, any contemporary artist trying to represent Christ is unlikely to be seeking either to portray a figure from 2,000 years ago or to produce a devotional image.

What might prompt such an artist to undertake this task is likely to be just the tension we have been sketching: how do you show transcendent difference in the representation of earthly events? Once again, it is the question of the iconoclastic controversy: how do you depict a humanity that is inseparably united with divine action? For the artist as artist, it is a formal rather than a theological challenge, but all the more provocative for that.

What makes it absorbingly interesting to the theologian is that it is one way of expressing the central issue of all talk about Jesus Christ. To say that Jesus is God made flesh is, strictly, to say that you cannot represent God as something added to the flesh, or as "flesh" somehow strained to extremity. This is simply to paraphrase the formulae of traditional doctrine: Jesus is complete in divinity, complete in humanity. To show divinity in such a context has just the same problems as to speak of it: it is not an added element in a compound but a characterisation of the whole. And thus it is speakable or tangible only as perceived in the changes it effects.

The divinity of Christ cannot be shown in a depiction of Jesus "alone", in a photographic historical portrait. The attempt to do this in 19th-century religious art by a rather specious realism coloured by devotional emotion is not going to work for just the reasons that revisionist doctrines of the same period have not worn well: divinity is not a human with inspirational extras. So we are driven back to looking for the ways in which this ordinary human identity alters the lines of force in the field - spiritual, moral, visual.

If this is right, the Christian will learn something of major importance by watching how the artist deals with the challenge of representing divine action in the historical world. And perhaps, too, the artist's continuing fascination with the challenge of depicting Christ is because there is something of almost paradigmatic seriousness in the challenge.

Any artist is going to be in the business of showing the world differently. The question is always how that showing creates an environment, a continuous world, in such a way that it makes still more difference possible in the world it started from. Or, in plainer English, how it communicates sufficiently to enlarge the world.

The event of Jesus Christ is for the Christian tradition the unsurpassable enlargement of the world. It makes possible what had been impossible. And it does so without tearing the fabric of history and matter. Put like that, it becomes clear why representing miracles is pretty hard, and representing the Resurrection the most difficult thing of all (how many effective pictures are there of the Resurrection? I'm inclined to say only one, Piero's in Borgo San Sepolcro - by refusing to dramatise, to show a series of happenings, but offering a single moment of monumental and timeless relation between God and sleeping humanity, and a face of incomparable stillness, a pivot on which all movement moves).

How does presence alter things? That is what the artist tackling this most impossible of tasks is after; it is both the extreme case of every artistic challenge and something that cannot be addressed except in relation to these particularities, these stories, this face and figure. We watch expectantly as the artist searches for the appropriate form of the uncanny, waiting to see if our world becomes strange as a result of having this particular stranger, Jesus, introduced into it.